Optical systems are presently being employed in the communication of voice and video information as well as in the high-speed transmission of data. Optical communication systems are desired because of the wide bandwidth available for the information signal channels.
Although this wide bandwidth is available, many of the existing optical fiber systems use only a single channel per optical fiber. Typically, this channel is transmitted at a wavelength in the 1500 nm band in one direction from a transmitting end to a receiving end and requires a second optical fiber to achieve bi-directional communication; however, recent increase in telecommunications traffic has resulted in a need for further fiber resources. One way this need was met, was to install additional optical fiber cables. Another was to increase the number of channels carried by same fibers.
Recently, technologies that can add additional channels to existing optical fiber cables already in the ground, have gained acceptance. These technologies seek to provide more than one channel on a single existing optical fiber and are therefore aimed at enhancing the efficiency of the existing fiber optic cable network. These technologies include wavelength division multiplexing (WDM) and bidirectional transmission.
When a number of wavelengths are multiplexed and transmitted on a single optical fiber, customarily, these channels must later be demultiplexed into separate channels or wavelengths of light. For example, it may be cost effective to transmit signals of wavelength .lambda.1, .lambda.2, .lambda.3, .lambda.4, .lambda.5, and .lambda.6 (.lambda. denoting a wavelength, lambda) along a single optical fiber, however, demultiplexing means are required to separate the light into six separate channels. Of course, it is desired to perform this demultiplexing at a minimum cost and with as little signal loss as possible. Furthermore, if signal loss exists, it is important for any signal loss present on any channel to be of a similar magnitude for all channels being demultiplexed.
There are several technologies that can be used to construct WDM filters. For example, etalon technology, diffraction grading technology, fused biconic taper technology, and holographic filter technology. One technology that has proven to be widely useful in the telecommunications industry is dichroic filter technology. This technology offers wide channel passbands, flat channel passbands, low insertion loss, moderate isolation, low cost, high reliability and field ruggedness, high thermal stability, and moderate filter roll-off characteristics.
An illustrative example of a conventional three-port dichroic filter 300 is shown in prior art FIG. 3. A dichroic filter is comprised of one or more layers of dielectric material coated onto a, for example, glass substrate 305 with lenses 310 to focus the incoming and outgoing optical signals. The choice of dielectric material, the number of dielectric layers coated onto the substrate, and the spacing of these layers are chosen to provide the appropriate transmissive and reflective properties for a given "target" wavelength. For example, if .lambda.1 is the target wavelength to be transmitted through the filter, the number and spacing of the dielectric layers on the substrate 305 would be chosen to provide (1) a specified passband tolerance around .lambda.1 and (2) the necessary isolation requirements for all other transmitted wavelengths, for example, a wavelength, .lambda.2, transmitted by a second transmitter.
The dichroic, or WDM, filter is constructed by placing self-focusing lenses, such as "SELFOC" lenses 310, on either side of the dielectric substrate 305. "SELFOC" lens 310 collimates incoming light (.lambda.1 and .lambda.2) at the dielectric substrate.
Attached to the "SELFOC" lenses through an adhesive bonding process are, typically, single-mode optical fibers. For convenience, the locations at which optical fibers attach to the "SELFOC" lenses 310 are called ports: port 1 320, port 2 325, and port 3 330. Connected to the ports are optical fibers 335, 340, and 345 respectively.
For example, all of the light (comprised of .lambda.1 and .lambda.2) passing through fiber 335 connected to port 1 320 is collimated by lens 310 at the dielectric substrate 305.
Since the substrate is coated to pass wavelengths around .lambda.1, virtually all of the light at .lambda.1 passes through the dielectric substrate 305 and, via the second "SELFOC" lens, is focused into port 3 330, and passes away from the filter on optical fiber 345. Any other wavelength incident on the filter through port 1 320 (e.g., light of wavelength .lambda.2) is reflected off the multi-layer substrate, focused back through the first "SELFOC" lens to port 2 325, and passes away from the filter on optical fiber 340. Likewise, the filter performs the same function for light traveling in the opposite direction.
Heretofore, it has been common practice, to sequentially arrange or cascade optical filters such that a first, second, third, . . . and nth wavelength are removed or separated from an optical signal comprising n wavelengths or channels, sequentially by n cascaded optical filters. Generally, after a first wavelength or channel is removed, the remaining n-1 channels are reflected backward to the remaining n-1 cascaded filters. Subsequently after a second wavelength or channels is removed, the remaining n-2 channels are reflected backward to the remaining n-2 cascaded filters, and so on. Of course it is well known that as the nth wavelength of the optical signal propagates along such a chain of n filters, signal power loss occurs along the chained path. This signal power loss is a result of both the overall distance that the signal must travel, and, more importantly much of the power loss occurs at each GRIN lens fibre interface or port.
Hence, channel 2 which must encounter two filter elements prior to be being removed or demultiplexed from the multiplexed signal channels undergoes less loss, than for example, channel 16 which encounters 16 filter interfaces.
As was stated heretofore, preferably, when a demultiplexor separates a group of channels into individual channels losses for each channel should most importantly be minimal and it is preferable that any losses introduced by the system should be as near to equal as possible for all channels. This is not the case with conventional demultiplexor designs using a conventional arrangement of cascaded narrow band dichroic filters.
It is therefore an object of this invention to provide a system for multiplexing and demultiplexing wherein overall signal loss is minimized.
It is a further object of this invention, to lessen the effects unequal loss due to sequentially removing n channels, one at a time in a conventional, sequential manner.
It is also an object to provide a filtering system wherein less expensive filters having poor (less steep) slopes can be used to separate groups of channels having suitable channels spacing between said groups.
It is also an object of the invention, to first provide suitable channel spacing between groups of channels such that inexpensive wide band filters can be used to separate the groups of channels prior to demultiplexing said groups.
It is an object of this invention to provide a system wherein sequential optical channels having a 100 Ghz channel spacing or less can be demultiplexed using conventional dichroic optical filters.